To make Postfix send from a specified IP address, first make sure the IP address is present on the server and routable (i.e. has right subnet, gateway, and is enabled). Then add the following to /etc/postfix/main.cf:
and make sure you reload the configuration:
smtp_bind_address=the.new.ip.address
and make sure you reload the configuration:
/etc/init.d/postfix reload